A novel low-pressure injection molding technique for fabricating anode supported solid oxide fuel cells
Introduction
In the past few years, solid oxide fuel cells (SOFCs), with high conversion efficiency, low pollution emission, economical cost, and flexible fuel applicability, have been developed as one of the most promising electricity generation technologies [1], [2], [3], [4]. Generally, there are three main SOFC configurations: electrolyte-supported, cathode-supported, and anode-supported, among which, anode-supported SOFC has attracted the most attention because of its high output power density and low ohmic and polarization resistance losses [5], [6], [7]. The anode-supported SOFCs are with a typical sandwich structure consisting of porous anode substrate, dense electrolyte membrane, and porous cathode thick film. Fabricating the porous anode substrates is one of the key processes in preparing the anode-supported SOFCs. The anode substrates mainly provide three functions: supporting the electrolyte membrane, collecting and electrical current, and providing anode reaction sites. Therefore they need to have sufficient mechanical strength, reasonable electronic conductivity, and proper porosity and sufficient triple phase boundaries (TPBs) [8], [9].
At present, the commonly used techniques for fabricating anode substrates are tape-casting, extrusion molding, and slip casting. Generally, tape-casting [10], [11] and extrusion molding [12] are respectively used to fabricate planar and straight tubular anodes, especially for mass production. However, they are not suitable for fabricating products with complex shape. Although slip casting can be used to make products with various complicated shapes and it has been applied in producing anode-supported SOFCs [13], [14], [15], it is difficult to use slip casting in preparing SOFC anode substrates of high quality. As slip casting is a relatively slow process, a highly stable slurry/suspension is necessary to guarantee the homogeneity of products (such as identical wall thickness). To achieve the required stability, one must tailor interparticle forces, through properly charging the particle surfaces, adjusting pH of the suspension, etc. [16]. While it may be easy to get a stable slurry with only one kind of particles, such as YSZ particles, it is difficult to maintain the stability of a slurry with two or more than two kinds of particles, such as in the case of SOFC anode slurry, with NiO, YSZ, and graphite (pore former) particles, because the conditions optimized for well suspension of one kind of particles might result in coagulation of the other kinds of particles.
Low pressure injection molding (LPIM) is a traditional ceramic forming technique [17], [18], [19]. Its basic process is schematically illustrated in Fig. 1. First, nonplastic ceramic powders and hot paraffin liquid are mixed together to get a hot suspension (above the melting point of paraffin) with low-viscosity. Then the slurry is immediately injected into a cold metal mould, cooled and solidified to form pieces with required shape. Finally, the green products are dewaxed and sintered. The LPIM technique is fast and cost effective, especially appropriate for fabricating complex-shaped and small-sized ceramic components.
Cone-shaped tubular anode-supported SOFC was proposed by Liu [20] for portable applications. With a series of cone-shaped tubular unit cells connected one another in an electrical series, a SOFC stack with compact volume and light weight can provide a relatively high voltage and power output. The cone-shaped tubular unit cell (or single cell) with the diameter of one end larger than that of the other, which is difficult to be made with traditional extrusion technique, can be fabricated with the LPIM technique.
In the presented work, for the first time, LPIM technique, which has been generally used to make dense alumina electronic ceramic components, is successfully developed to fabricate porous NiO–YSZ cone-shaped tubular anode substrates for anode-supported SOFCs. Several single cells and a two-cell-stack have been assembled and their electrochemical performances have been characterized.
Section snippets
Fabrication of NiO–YSZ cone-shaped tubular anode substrates by LPIM
Nickel oxide (NiO, Australia) and 8 mol% yttria-stabilized zirconia (YSZ, Building Material Academy of China) powders were pre-calcined at 1200 °C for 2 h. Then the NiO and YSZ, with a weight ratio of 3:2, were mixed with graphite through ball milling for 3 h with ethanol as medium. Five powder mixtures, with graphite of 0, 5, 10, 15, 20 wt.%, respectively, were prepared to investigate the effect of pore former on the porosity of the anode. After drying, the powder mixtures were respectively
Porosity and microstructure of the anodes fabricated with different amount of pore former
Generally, traditional LPIM technique is used to manufacture dense alumina electronic ceramic elements. The paraffin content of the slurry for injection is controlled as less as 12.5–13.5 wt.% to make the slurry flowable and guarantee the density of the sintered products. However, the anode substrates of SOFCs need to have proper porosity, thus cannot be fabricated in the traditional way. While increasing the paraffin addition may lead to some pore formation in the anode, too much paraffin
Conclusion
The low-pressure injection molding (LPIM) technique is fast, low-cost, and especially suitable for mass production of small and complex-shaped ceramic components with high accuracy in size and low deformation in shape. With 15 wt.% paraffin as plasticizer, porous anode substrates can be obtained, and larger porosity can be obtained and controlled by adding proper amount of graphite as pore former in preparation. Acceptable performance can be obtained with SOFCs fabricated with none or 5 wt.%
Acknowledgements
The authors are grateful to the National Science Foundation of China (NSFC, Grant No. 20276097).
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2019, Ceramics InternationalCitation Excerpt :There have also barely been any reports about SOFC stacks with the PIM technique applied. In the previous work [3], the feasibility of the LPIM technique for fabricating porous NiO-YSZ anode supports for tubular anode-supported SOFCs was successfully demonstrated. The advantages of the LPIM technique are easy operation, equipment simplicity, high production efficiency, and producing near net shape.